Years ago aircraft electrical systems operated on low voltage DC power, for example, 6, 12 or 28 volts. However, the burgeoning use of electrical instrumentation such as radios increased the amount of power required on the aircraft. Oftentimes the added power demands could be managed by the low voltage DC power systems in spite of the modest capabilities of distribution systems. Soon, however, electric motors and other high power demand equipment began to appear on aircraft thereby making the use of low voltage DC power and the concomitant distribution system impractical. For example, a modest amount of power such as 6 kilowatts (KW) required large, heavy and inflexible cables to carry currents in excess of 200 amperes. To address these new power needs, AC aircraft power systems distributing power at 115 volts AC, three phase, 400 hertz (Hz) began to emerge. These AC systems distributed and switched power easily. Furthermore, the use of 400 Hz allowed electromagnetic devices such as motors and transformers to remain of modest size and weight.
While most instruments operate on 115 VAC and 400 Hz, during emergencies and when no engines are running, power is supplied by batteries. DC power is therefore needed to keep these batteries charged and also to supply power to DC devices on the aircraft, e.g., engine starters. Currently the most widespread means of this conversion is the transformer rectifier unit (TRU).
The purpose of a TRU is to convert AC power to DC. For the purposes of the description herein, AC power is taken to be 115 volts, line to neutral, three phase 400 Hz. DC power is taken to be 28 volts DC. These values are typical in aerospace applications and as such may a depending on the desired end use application. Such a TRU 100 is shown in FIG. 1 (FIG. 1) and represents a configuration that has been well known in the art for decades. The three phase AC input is connected to a transformer primary 101 that may be wound in either a star or a delta configuration. There are two secondary windings, one star 102 and one delta 103 which are connected to six pairs of rectifiers 104 to provide 28 volts. Rectifying the six phases from the star and delta windings leads to a small ripple voltage that is 100 (1-cos 15)% of the output which corresponds to 0.95 volts peak to peak for a 28 volt system.
This TRU configuration has become a standard throughout the aerospace industry for several reasons. First, the 400 Hz AC frequency of aircraft systems affords the use of a small, light transformer. Second, the system is inherently very simple. Third, the semiconductor diodes used as rectifiers are rugged and can withstand very high overloads for sufficient time to clear circuit breakers or start aircraft power units. Fourth, due to the combination of the star and delta wound secondaries the output ripple is inherently low, thereby obviating the need for large filter capacitors. Fifth, the input power factor (a measure of the phase angle difference between input current and input voltage and also a measure of currents at harmonics of the input frequency) is close to unity with low input harmonic distortion. Last, the efficiency of the system is high. However, a drawback of this TRU configuration is that the output voltage is not regulated. It varies with the variations of input voltage and the output load, as increasing current causes increasing voltage drops across the transformer windings.
Regulating a TRU is no easy task if its beneficial features are to be retained. Electronic power supplies that have a unity power factor AC input and a regulated DC output are commonly available, but do not have the high efficiency of the TRU--90% and higher--nor do they provide high output currents that are ten times the rated output and greater.
One system that is presently in use has SCR (silicon controlled rectifier) switches in the transformer primary circuit in order to regulate the transformer input voltage. This particular system manufactured by Leland Electrosystems Inc. (Moore et al. U.S. Pat. No. 5,541,830) provides good regulation and will supply the required high overload currents but is inefficient, has poor input power factor and high input current harmonics. Known AC/DC power converters have utilized TRU's comprising a transformer having a primary winding which receives input AC power and one or more secondary windings which are coupled to one or more rectifier circuits. The rectified transformer output is thereafter filtered and regulated to produce DC output power. None however, embody the regulation means taught presently.
One type of regulated AC/DC power converter is disclosed in Terry et al. U.S. Pat. No. 2,642,558. This power converter includes a transformer having a primary winding coupled to an AC source and multiple secondary windings, one of which is coupled to a rectifier bridge for providing approximately half the desired output voltage at full load. A pair of additional secondary windings are coupled to the anodes of a pair of gas discharge tubes, the cathodes of which are coupled together to an output filter of the device. The gas discharge tubes are fired at a variable point in each half cycle of the AC waveform to in turn supply the proper additive voltage to the output of the full wave bridge so that the output voltage is maintained at a regulated value.
Yenisey U.S. Pat. No. 3,270,270 discloses a regulated power supply wherein an input AC power source is coupled to a first transformer primary winding. The AC power source is further coupled through a controlled switch to a second primary winding of the transformer. A pair of secondary windings of the transformer are coupled through rectifiers to output terminals of the device. A main portion of the output power is provided through the first primary winding while intermittent or variable power modulated by the controllable power switch is provided through the second primary winding and combined with the main output power to regulate the same. In a further embodiment of the power supply, intermittent power is provided through each of a pair of primary windings, one of which adds intermittent power to the unregulated power and the second of which subtracts intermittent power from the unregulated power.
Another regulated AC/DC power supply is disclosed in Higuchi et al. U.S. Pat. No. 4,232,363. Higuchi et al. teach a closed loop regulated AC-DC power supply comprising a power transformer having two secondary windings and a compensating transformer having two compensating windings individually in a series circuit with the principle power supply secondary windings. The other secondary windings on the power transformer supply exciting windings for compensating variations in output potential by control of the compensating transformer circuitry which has an input connected across the load and an output for varying the AC supply to the exciting windings inversely of variations in load potential. A differential amplifying circuit is used for determining the load potential error and driving a transistor effectively to insert a DC component in inverse feedback relationship into the exciting windings of the compensating transformer.
Glennon et al. U.S. Pat. No. 4,739,466 teaches a regulated AC/DC converter including a transformer having a primary winding which receives AC input power and a secondary winding, a rectifier coupled to the secondary winding for rectifying the output of the same, a switching circuit coupled to the rectifier for developing trim voltage which is combined with the rectified output of the secondary winding to develop voltage-regulated DC output power and a switch control circuit for controlling the switching circuit. The switching circuit develops trim voltage which aids the rectified secondary winding output when such output is below a desired level and which opposes the rectified secondary winding output when such output is above the desired level.
A boost converter power supply is disclosed in Bassett et al. U.S. Pat. No. 5,446,366. Bassett et al. teach a boost converter power supply circuit that uses energy from a diode recovery current which flows in the blocking diode at diode commutation to discharge the capacitance associated with the main switch of the boost converter in order to achieve zero voltage switching thereby reducing high frequency ripple. The diode recovery energy is initially captured in an inductor, is then transferred to a capacitor, and is then transferred back to the inductor prior to the main switch being switched on. Furthermore, a control circuit for controlling a boost converter to achieve power factor correction is disclosed which does not use a high gain input current feedback loop.
Vinciarelli et al. U.S. Pat. No. 5,786,992 teaches a power conversion apparatus consisting of boost conversion elements for effecting a boost conversion of power originating from a varying voltage source; additional conversion elements for also effecting conversion of power originating from the varying voltage source; and circuitry for selectively configuring the conversion elements with respect to delivery of power to a load to achieve greater conversion efficiency than if all of the power delivered to the load were constrained to flow through the additional conversion elements and at least some of the boost conversion elements. In another aspect power is converted from a voltage source having a predetermined source voltage range, and delivered to a load via two sets of power conversion elements, each set for effecting conversion of power from the voltage source, at least one of the sets having an input operating voltage range narrower than the source voltage range. The apparatus includes control circuitry for configuring and reconfiguring interconnections of the two sets with the voltage source and the load to provide an input operating voltage range for the apparatus which is greater than the input operating range of the set of power conversion elements having the narrower input operating voltage range. In another aspect, the input voltage operating range is enhanced for a power conversion circuit which has two sets of power conversion elements arranged to share power delivered to a load. The circuit is selectively configured with the two sets in series or not in series in a manner so that the input voltage operating range of the power conversion circuit is broader than the input voltage operating range of either of the sets of power conversion elements.
Utilizing flyback technology, Cross U.S. Pat. No. 5,796,595 teaches an interleaved flyback electrical power converter system having a plurality of flyback power converters operated in continuous mode, each converter utilizing a power switch operated under zero voltage switching conditions. The interleaved flyback power converter system is suitable for high power, high frequency applications. However, Cross requires that multiple large transformers be series connected to appreciably increase efficiency.
Wilcox et al. U.S. Pat. No. 5,847,554 teaches synchronous switching regulator circuits with voltage-drop sensing circuitry in which the current sensing element typically in series with a load is eliminated. Voltage drops are measured across the regulator's synchronous switching element, and, in some cases, also across the regulator's main switching element. Measured voltage drops are used to derive a current analog signal indicative of the amount of current supplied by the regulator. The current signal is then compared with a threshold value to determine whether the regulator's duty cycle should be varied.
Another AC/DC power supply is disclosed in Slack et al. U.S. Pat. No. 5,864,473. Slack et al. teach a power supply that contains two stages which convert AC power to a low-voltage, high-current DC output. The first stage is an AC to DC converter using a full wave bridge rectifier to convert the voltage from the AC source to an intermediate raw DC voltage. The second stage is a DC to DC forward converter using a transformer with its primary winding connected to the intermediate raw DC voltage through a switch which is cycling "on" and "off" at a predetermined frequency. During the switch "on" portions of the cycle a current flows in the primary winding powering the output and charging an inductor which is connected to the secondary winding. When the switch is turned "off", no current flows through the transformer and the load is powered by the energy stored in the inductor. A secondary coil is magnetically coupled to the inductor and has an induced voltage proportional to the output voltage. An IC controller is used to control the duty cycle of the switch based on the voltage in the secondary coil which determines the power supply output. The duty cycle of the switch is varied in response to variations in the load.
Another flyback design is disclosed in Moreau U.S. Pat. No. 5,867,374. Moreau teaches a method and apparatus for providing a rectified voltage to a flyback switch-mode supply circuit with power factor correction. The invention comprises a rectifying bridge and a charge path for charging a capacitor from the rectified voltage obtained from the output of the bridge. The degree of power factor correction is achieved by selecting a time at which either the rectifying bridge output or the capacitor Provides a voltage to the supply circuit. The input to the supply circuit is a primary winding having n1 turns between an upper and intermediate tap, and n2 turns between an intermediate and lower tap. The capacitor is connected to the upper tap via a diode, a first rectifying bridge output is connected to the intermediate tap via a diode, and a second rectifying bridge output is connected to the lower tap via a controllable switch. When the rectified voltage is greater than a threshold voltage related to the peak rectified voltage by the ratio n2/(n1+n2), the rectifying bridge outputs provide the voltage to the supply circuit. When the rectified voltage is lower than this threshold voltage, the capacitor provides the voltage to the supply circuit. In this manner, the timing of sources providing voltage to the supply circuit and hence the degree of power factor correction is determined by the ratio of the primary winding turns n1 and n2. Moreau's rationale behind regulation is to account for varying supply voltage rather than varying output load as is taught.
Hwang et al. U.S. Pat. No. 5,903,138 teaches a two-stage switching regulator having low power modes responsive to load power consumption. A first stage of the regulator is a power factor correction (PFC) stage. A second stage is a pulse-width modulating (PWM) stage. When a load draws a high level of current, switching in both stages is enabled. Under certain conditions, switching in either one or both of the stages is disabled by the duty cycle for the corresponding switch falling to zero. When switching in both stages is enabled, as in a normal mode, switching in the PWM stage is then disabled in response to an error signal falling below a first error threshold. When switching in the PFC stage is enabled and switching in the PWM stage is disabled, as in a first low power mode, switching in the PFC stage is then disabled in response to a feedback voltage rising above a first feedback threshold. When switching in both stages is disabled, as in a second low power mode, switching in the PWM stage is then enabled in response to the error voltage rising above a second error threshold, higher than the first error threshold. When switching in the PFC stage is disabled and switching in the PWM stage is enabled, as in a third low power mode, switching in the PFC stage is enabled in response to the feedback voltage falling below the second feedback threshold. The rationale behind Hwang et al.'s multi-state regulation is clearly to conserve power in periods of low current draw.
In view of the foregoing, clearly there exists a need for an improved DC power supply that addresses the shortcomings of the prior art, e.g., poor efficiency and high complexity. Such improvements serve not only to enrich the art but also to provide a more efficient and reliable power supply for particularly the aerospace industry.